Endopolygalacturonases (EC 3.2.1.15) produced by Stereum purpureum (Chondrostereum purpureum), a pathogenic fungus, that causes apple silver leaf disease, are unique proteinous phytotoxins responsible for inducing the same symptoms of silver leaf disease as those in naturally infected trees. 1) As endopolygalacturonase (endoPG) is an enzyme that catalyzes hydrolysis of the α 1,4 glycosidic bond of polygalacturonic acid, cleavage of pectin in the cell wall by endoPG is assumed to cause formation of an air layer between epidermal cells and the palisade parenchyma, resulting in the silver or lead color of the leaves.The S. purpureum ASP 4B isolate used in this study produces EndoPG I, II, III and IV, each exhibiting a different isoelectric point.2) Considering it makes up over 50% of EndoPGs produced in this isolate, the polygalacturonase with the highest isoelectric point (EndoPG I) has to date been studied in most detail.3,4) EndoPG I has been separated into three components (Ia, Ib and Ic) by CM52 column chromatography in spite of an identical isoelectric point.5) The number of sugar chains (mostly M5 high mannose type) for each EndoPG I has been shown as two for Ia, three for Ib, and four for Ic.4) Enzymatic properties were found common between EndoPG Is except for the difference in number of sugar chains. A common characteristic found of EndoPG Is is their extreme thermostability, with no decrease in activity on heating at 70 C for 30 min.3) On amino acid sequence deduction by cDNA, 6,7) and ESI MS of the sugar chain free enzyme, a difference of 4498 Da was found between the molecular weights of both. With results of protein sequencing from the N terminal amino acid and cDNA cloning in agreement, the C terminus was judged to have differed due to C terminal analysis showing a leucine to be in place of a glycine.4) This result was supported by ESI MS analysis showing 44 amino acid residues to be missing from the C terminal region of the mature enzyme.4) For EndoPGs, this was the first report of its kind. The reasons for such an absence are yet unknown. The position of three disulphide bonds in EndoPG I has also been clarified based on ESI MS analysis of peptides from tryptic digestion of the enzyme.4) Structural analysis by X ray crystallography has given results at resolutions up to 0.96 A . 8) Part of the enzyme s catalytic mechanism has also been clarified by analysis of the crystal complex (binary and ternary) occurring in reaction between enzyme and substrate.9,10) EndoPG I with PG activity has also been successfully expressed in Echerichia coli 11) and Pichia pastoris. 12)In such a way, characteristics of EndoPG I have been